Process for controlling wheel longitudinal force in vehicle

ABSTRACT

A total longitudinal force which is a sum total of the longitudinal forces applied to the plurality of wheels is detected or determined, and the longitudinal forces applied to the wheels are controlled on the basis of a front wheel-side target wheel longitudinal force and a rear wheel-side target wheel longitudinal force determined by distribution of the total longitudinal force at predetermined distribution proportions. When the slipping of a wheel is detected, the distribution proportions are changed, so that the target wheel longitudinal force is larger on one of the front and rear wheel sides in which more wheels are in non-slipping states.

This application is a continuation of application Ser. No. 08/252,558filed Jun. 1, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for controlling the brakingforce or driving force as a wheel longitudinal force in a vehicle inwhich longitudinal forces applied to a plurality of wheels can becontrolled at least on a front wheel side and on a rear wheel side.

2. Description of the Related Art

There is a process in which the braking force as the wheel longitudinalforce can be controlled at least on the front wheel side and on the rearwheel side, and which is conventionally known, for example, fromJapanese Patent Application Laid-open No. 237252/89.

In the above prior art process, the braking forces for the left andright front wheels are controlled collectively, and the braking forcesfor the left and right rear wheels are controlled independently. Whenthe frictional coefficient of a travel road surface is different on theleft and right sides of the vehicle, when the carried load variation islarge, as well as when there is a difference in capacity among tires offour wheels (for example, when studless tires are mounted on onlydriving wheels, when one of left and right wheels is failed to bebraked, or when a temper tire is mounted on a certain wheel), ananti-lock control (which will be referred to as an ABS controlhereinafter) of any wheel liable to slip is started during braking andat this time point, a driver of the vehicle feels that the ABS controlis executed. Therefore, even if a margin is left in the braking forcefor other wheels, the drive cannot further depress a brake pedal in manycases, and cannot obtain a maximum braking capacity for each wheel.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process forcontrolling the wheel longitudinal force in a vehicle, wherein thecapacity for the wheel can be exhibited to the maximum, without changeof the total wheel longitudinal force corresponding to an operation bythe vehicle driver.

To achieve the above object, according to the invention, a totallongitudinal force, which is a sum total of the longitudinal forcesapplied to the plurality of wheels, is detected or determined, and thelongitudinal forces applied to the wheels are controlled on the basis ofa front wheel-side target wheel longitudinal force and a rear wheel-sidetarget wheel longitudinal force determined by distribution of the totallongitudinal force at predetermined distribution proportions. When theslipping of the wheel is detected, the distribution proportions arechanged, so that the target wheel longitudinal force is larger on one ofthe front and rear wheel sides in which more wheels are in non-slippingstates. Therefore, while the total wheel longitudinal force required bya vehicle occupant is made invariable, the maximum wheel longitudinalforce can be exhibited within such range.

According to the present invention, in addition to the foregoing, inchanging the distribution proportions, the change amount at the timewhen the front wheel-side target wheel longitudinal force is increasedis determined larger than the change amount at the time when the rearwheel-side target wheel longitudinal force is increased. Therefore, thefrequency of occurrence of slipping of the wheel is larger on the frontwheel side, which can contribute to an improvement in stability of thevehicle.

In addition, the longitudinal forces applied to the plurality of wheelsare controllable independently, and loads shared to the wheels in astopped state of the vehicle are determined. The apparent direction andamount of displacement of the center position of gravity of the vehicleis calculated on the basis of a longitudinal acceleration of a lateralacceleration of the vehicle, and the apparent center of gravity positionof the vehicle in accordance with apparent direction and amount ofdisplacement is determined. When the slipping of the wheel is detected,the apparent center of gravity position of the vehicle is corrected sothat it is displaced away from such wheel on a straight line connectingthe wheel which is in the slipping state and the apparent center ofgravity position of the vehicle, as viewed in a plane, and thedetermined shared load is corrected on the basis of a corrected apparentcenter of gravity position of the vehicle. Distribution proportions ofthe wheel longitudinal force are determined for every wheel on the basisof corrected shared loads. Therefore, when the distribution proportionsare changed, it is possible to disperse the increment at a properdistribution to the wheels having an increased distribution proportion.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described by way of an embodiment inconnection with the accompanying drawings, in which

FIG. 1 is a diagram illustrating a braking system for a vehicle;

FIG. 2 is a block diagram illustrating an arrangement of a controlsystem;

FIG. 3 is a graph illustrating an established map of the total hydraulicbraking pressure according to the brake depression force;

FIG. 4 is a diagram for explaining the apparent displacement of thecenter position of gravity in a longitudinal direction of the vehicle;

FIG. 5 is a diagram for explaining the apparent displacement of thecenter position of gravity in a lateral direction of the vehicle;

FIG. 6 is a diagram for explaining the apparent variation in centerposition of gravity on X and X coordinates;

FIG. 7 is a block diagram illustrating an arrangement of a forcedgravity center position-displacement amount calculating means;

FIG. 8 is a diagram for explaining the forced displacement of the centerposition of gravity on the X and Y coordinates;

FIG. 9 is a graph illustrating an established map of weightingcoefficient;

FIG. 10 is a graph illustrating an established map of time constant;

FIG. 11 is a diagram illustrating one example of a variation indisplacement amount on X and Y axes in accordance with a brakingoperation;

FIG. 12 is a diagram illustrating a failure diagnosis map;

FIG. 13 is a graph illustrating the correction rate corresponding to thevehicle speed;

FIG. 14 is a graph illustrating the correction rate corresponding to theX coordinate of the center position of gravity after forceddisplacement;

FIG. 15 is a graph illustrating the correction rate corresponding to theY coordinate of the center position of gravity after forceddisplacement;

FIG. 16 is a block diagram illustrating an arrangement of a yaw controlquantity calculating means;

FIG. 17 is a graph illustrating the reference yaw rate corresponding tothe vehicle speed;

FIG. 18 is a graph illustrating the correction rate corresponding to thevehicle speed;

FIG. 19 is a graph illustrating the correction rate corresponding to thelongitudinal acceleration; and

FIG. 20 is a graph illustrating the correction rate corresponding to thelateral acceleration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 1, a right front wheel brake B_(FR) is mountedto a right front wheel W_(FR) of a four-wheel vehicle which is of afront engine and front drive type (FF type). A left front wheel brakeB_(FL) is mounted to a left front wheel W_(FL). A right rear wheel brakeB_(RR) is mounted to a right rear wheel W_(RR), and a left rear wheelbrake B_(RL) is mounted to a left rear wheel W_(RL). The brakes B_(FR),B_(FL), B_(RR), B_(RL) are of the same specification.

A tandem type master cylinder 1 includes a pair of output ports 1a and1b. The output port 1a is connected to the right front wheel brakeB_(FR) through a modulator 2_(FR) capable of controlling the fluidpressure and also to the left rear wheel brake B_(RL) through amodulator 2_(RL). The other output port 1b is connected to the leftfront wheel brake B_(FL) through a modulator 2_(FL) and also the rightrear wheel brake B_(RR) through a modulator 2_(RR).

The operations of the modulators 2_(FR), 2_(FL), 2_(RR), 2_(RL), i.e.,braking fluid pressures applied to the brakes B_(FR), B_(FL), B_(RR),B_(RL) are controlled independently by a control unit C.

Referring to FIG. 2, connected to the control unit C are a depressionforce detecting sensor 3 for detecting a brake depression force F_(B) asa quantity of braking operation by a brake pedal (not shown), wheelspeed sensors 4, 5, 6, 7 for detecting wheel speeds, V_(FR), V_(FL),V_(RR), V_(RL) of the wheels W_(FR), W_(FL), W_(RR), W_(RL),respectively, a longitudinal acceleration detecting sensor 8 fordetecting a longitudinal acceleration G_(SX) of the vehicle, a lateralacceleration detecting sensor 9 for detecting a lateral accelerationG_(SY), a steering angle detecting sensor 10 for detecting a steeringangle Θ as a quantity of steering operation by a steering handle (notshown), a yaw rate detecting sensor 11 for detecting a yaw rate Y_(A) asa quantity of vehicle turned actually, and a braking fluid pressuresensor 12 for detecting a braking fluid pressure P_(B).

The control unit C includes a total longitudinal force determining means13 for determining a total braking fluid pressure P_(T) for all fourwheels on the basis of a detection value detected by the depressionforce detecting sensor 3, a deceleration correcting means 14 forcorrecting the total braking fluid pressure P_(T) determined in thetotal longitudinal force determining means 13 by a deceleration controlquantity P_(G) to provide a first corrected total braking fluid pressureP_(T1), a gain correcting means 15 for applying a gain correction to thefirst corrected total braking fluid pressure P_(T1) t provide a secondcorrected total braking fluid pressure P_(T2) a vehicle speedcalculating means 16 for calculating a vehicle speed V on the basis ofwheel speeds V_(RR), V_(RL) of the right and left rear wheels W_(RR),W_(RL) which are follower wheels, a slip detecting means 17 fordetecting which wheel is fallen into a slipping state during braking onthe basis of the speeds V_(FR), V_(FL), V_(RR), V_(RL) of the wheelsW_(FR), W_(FL), W_(RR), W_(RL), a gravity-center position calculatingmeans 18 for calculating the direction and quantity of apparentdisplacement of the center position of gravity of the vehicle on thebasis of a longitudinal acceleration G_(SX) and a lateral accelerationG_(SY), a gravity-center position forced-displacement quantitycalculating means 19 for calculating an amount of forced displacement ofthe center position of gravity to correct the apparent center of gravitydetermined in the gravity-center position calculating means 18 when theslipping of the wheel has been detected by the slip detecting means 17,a yaw control quantity calculating means 21 for calculating a yawcontrol quantity Y_(C) on the basis of the total braking fluid pressureP_(T), the vehicle speed V, the longitudinal acceleration G_(SX), thelateral acceleration G_(SY), the steering angle Θ and the detection yawrate Y_(A), a shared-load proportion calculating means 22 forcalculating shared-load proportions R_(FR), R_(FL), R_(RR), R_(RL) forthe four wheels on the basis of the calculated quantities determined inthe gravity-center position forced-displacement amount calculating means19 and the yaw control quantity calculating means 21, right front, leftfront, right rear and left rear wheel braking fluid pressure calculatingmeans 23 23_(FL), 23_(RR), 23_(RL) for calculating target braking fluidpressures P_(FR), P_(FL), P_(RR), P_(RL) of the wheel brakes B_(FR),B_(FL), B_(RR) and B_(RL) as target longitudinal forces for the wheelson the second corrected total braking fluid pressures P_(T) and theshared-load proportions R_(FR), R_(FL), R_(RR), R_(RL), respectively,and drive means 24_(FR), 24_(FL), 24_(RR) and 24_(RL) for driving themodulators 2_(FR), 2_(FL), 2_(RR), 2_(RL) on the basis of the targetbraking fluid pressures P_(FR), P_(FL), P_(RR), P_(RL), respectively.

The total longitudinal force determining means 13 determines a totalbraking force which is a sum total of wheel longitudinal forces appliedto the four wheels in accordance with the brake depression force F_(B).When the brakes B_(FR), B_(RR), B_(RL), B_(RL) of the same specificationare mounted to the four wheels W_(FR), W_(FL), W_(RR), W_(RL),respectively, braking forces exhibited by the brakes B_(FR), B_(FL),B_(RR), B_(RL) are proportional to the braking fluid pressurescontrolled by the modulators 2_(FR), 2_(FL), 2_(RR), 2_(RL),respectively, and it is possible to calculate the total braking force asthe total longitudinal force in the term of a total braking fluidpressure. Therefore, the total braking fluid pressure P_(T) applied tothe brakes B_(FR), B_(FL), B_(RR), B_(RL) is determined by the totallongitudinal force determining means 13 on the basis of the mappreviously defined in accordance with the brake depression force F_(B),as shown in FIG. 3.

The total braking fluid pressure P_(T) determined in the totallongitudinal force determining means 13 is supplied to the targetdeceleration determining means 25, where a target deceleration G₀ isdetermined in accordance with the total braking fluid pressure P_(T).The vehicle speed V, determined in the vehicle speed calculating means16, is supplied to a differentiating means 26, where a vehicledeceleration is determined by differentiating the vehicle speed. Suchvehicle deceleration and the target deceleration G₀ are supplied to acontrol quantity calculating means 27, where a deceleration controlquantity P_(G) is calculated on the basis of a deviation between thetarget deceleration G₀ and the detected vehicle deceleration.

The total braking fluid pressure P_(T) and the deceleration controlquantity P_(G) are supplied to the deceleration correcting means 14,where the deceleration control quantity P_(G) is added to the totalbraking fluid pressure P_(T) to provide a first corrected total brakingfluid pressure P_(T1).

The longitudinal acceleration G_(SX) detected by the longitudinalacceleration sensor 8 and the lateral acceleration G_(SY) detected bylateral acceleration sensor 9 are supplied to the gravity-centerposition calculating means 18. If coordinates of the center position ofgravity of the vehicle in a resting state are represented by (G_(X0),G_(Y0)), the gravity-center position calculating means 18 calculates thedirection and amount of apparent displacement of the center position ofgravity with a variation in load and also calculates coordinates (G_(X),G_(Y)) indicating an apparent displaced point of the center of gravityposition on the basis of such calculated value.

Referring to FIG. 4, if the height of the center of gravity positionfrom a grounded road surface is represented by H, and a force of gravityG is equal to 1, the apparent amount ΔX of displacement of the center ofgravity position in the lateral direction of the vehicle, i.e., in an Xdirection is determined according to ΔX=G_(SX) ×H.

Referring to FIG. 5, if the height of the center of gravity positionfrom a grounded road surface is represented by H, and a force of gravityG is equal to 1, the apparent amount ΔY of displacement of the center ofgravity position in the lateral direction of the vehicle, i.e., in a Ydirection is determined according to ΔY=G_(SY) ×H.

Further, if the total weight of the vehicle is represented by WT_(T) ;the shared loads of the right front wheel W_(FR), the left front wheelW_(FL), the right rear wheel W_(RR), and the left rear wheel W_(RL) inthe resting state of the vehicle are by WT_(FR), WT_(FL), WT_(RR),WT_(RL) WT_(T) (WT_(FR) =WT_(FR) +WT_(RR) +WT_(RL) +WT_(RL)),respectively; and the wheel base is by L_(B) and the tread is by L_(T),as shown in FIG. 6, the X coordinate G_(XO) of the center of gravityposition in the resting state is represented by a following expression:

    G.sub.XO ={L.sub.B ·(WT.sub.FR +WT.sub.FL) /WT.sub.T }-L.sub.T /2(1)

and the Y coordinates G_(YO) of the center position of gravity in theresting state is represented by a following expression:

    G.sub.yO ={L.sub.B ·(WT.sub.RR +WT.sub.RL) / WT.sub.T }-L.sub.T /2(2)

The X coordinate G_(X) of the apparent center of gravity position, as aresult of a variation in load during traveling of the vehicle, isrepresented by G_(X) =G_(XO) +ΔX, and the Y coordinate G_(Y) isrepresented by G_(Y) =G_(YO) +ΔY.

Referring to FIG. 7, the center of gravity position forced-displacementamount calculating means 19 calculates the amount of forced displacementof the center of gravity position in order to correct the apparentcenter of gravity position, so that the apparent center of gravityposition, determined by the gravity-center position calculating means 18upon detection of the slipping of the wheel by the slip detecting means17, is displaced away from the wheel on a straight line connecting thewheel in the slipping state with the apparent center of gravity positionof the vehicle as viewed in a plane. The center of gravity positionforced-displacement amount calculating means 19 includes an X- andY-axis displacement amount calculating section 28 for calculating anamount ΔD_(ABS-X) of displacement in a direction of the X axis and anamount ΔD_(ABS-Y) of displacement in a direction of the Y axis on thebasis of outputs from the slip detecting means 17, the center of gravityposition calculating means 18 and the braking fluid pressure detectingsensor 12, a frequency counter 29 for counting the frequency of slippingstates on the basis of the output from the slip detecting means 17 byincreasing the number of slipping states when the same wheel has beensuccessively fallen into slipping state and by decreasing the number ofslipping states when the different wheels have been irregularly falleninto slipping states, a weighting coefficient determining section 30 fordetermining a weighting coefficient k in accordance with an output fromthe frequency counter 29, a time constant determining section 31 fordetermining a time constant K in accordance with the braking fluidpressure P_(B) detected by the braking fluid pressure detecting sensor12, when the slipping state of the wheel has been detected by the slipdetecting means 17, a multiplying section 32 for multiplying theweighting coefficient k to the time constant K to provide a correctedtime constant K', a return calculating section 33 for calculating anamount ΔD_(ABS-X) of displacement in the direction of the X axis and anamount ΔD_(ABS-Y) of displacement in the direction of the Y axis at areturn time to return the forcedly displaced center position of gravitywith completion of the braking operation, failure diagnosis sections34_(X), 34_(Y) for diagnosing a failure on the basis of the correctedtime constant K' determined in the multiplying section 32 and an outputfrom the return calculating section 33, a braking operation detectingsection 35 for detecting a braking operation on the basis of an outputfrom the depression force detecting sensor 3, a switch circuit 36 foralternatively selecting either one of an output from the X- and Y-axisdisplacement amount calculating section 28 and an output from the returncalculating section 33 on the basis of the output from the depressionforce detecting sensor 3, and a center of gravity position determiningsection 37 for determining a center position of gravity in accordancewith an output from the center of gravity position calculating means 18and an output from the switch circuit 36.

In the X- and Y-axis displacement amount calculating section 28, theamount ΔD_(ABS-X) of displacement in the X-axis direction and the amountof ΔD_(ABS-Y) of displacement in the Y-axis direction are calculatedaccording to following expressions (3) and (4);

    ΔD.sub.ABS-X +ΔD.sub.ABS-X.sup.-1 +ΔX.sub.ABS-FR +ΔX.sub.ABS-FL +ΔX.sub.ABS-RR +ΔX.sub.ABS-RL (3)

    ΔD.sub.ABS-Y =ΔD.sub.ABS-Y.sup.-1 +ΔY.sub.ABS-FR +ΔY.sub.ABS-FL +ΔY.sub.ABS-RR +ΔY.sub.ABS-RL(4)

In the above expressions (3) and (4), each of ΔD_(ABS-X) ⁻¹, ΔD_(ABS-Y)⁻¹ is a last amount of displacement; each of ΔX_(ABS-FR), ΔX_(ABS-FL),ΔX_(ABS-RR), ΔX_(ABS-RL) is an amount of displacement in the X-axisdirection which is calculated when corresponding one of the right frontwheel W_(FR), the left front wheel W_(FL), the right rear wheel W_(RR),the left rear wheel W_(RL) has been fallen into its slipping state; andeach of ΔY_(ABS-FR), ΔY_(ABS-FL), ΔY_(ABS-RR), ΔY_(ABS-RL) is an amountof displacement in the Y-axis direction which is calculated whencorresponding one of the right front wheel W_(FR), the left front wheelW_(FL), the right rear wheel W_(RR) and the left rear wheel W_(RL) hasfallen into its slipping state.

Suppose that the slipping state of the left rear wheel W_(RL) has beendetected by the slip detecting means 17, as shown in FIG. 8. In thiscase, the apparent center of gravity position (G_(X), G_(Y)), attendanton a variation in load during traveling of the vehicle, is forcedlydisplaced away from the left rear wheel W_(RL) on a straight line Lconnecting the left rear wheel W_(RL) with the apparent center ofgravity position (G_(X), G_(Y)) of the vehicle, as viewed in a plane, bya forced displacement amount ΔD_(ABS-RL), which is calculated accordingto the following expression (5):

    ΔD.sub.ABS-RL =D.sub.ABS-R×(D.sub.TIRE -D.sub.G-RL)/D.sub.TIRE( 5)

Likewise, when the right rear wheel W_(RR), the left front wheel W_(FL)and the right front wheel W_(FR) have been fallen into their slippingstates, the apparent center of gravity position is forcedly displacedaway from the wheels W_(RR), W_(FL), W_(FR) on straight lines connectingthe wheels W_(RR), W_(FL), W_(FR) with the apparent center of gravityposition (G_(X), G_(Y)) of the vehicle, as viewed in a plane, by forceddisplacement amounts ΔD_(ABS-RR), ΔD_(ABS-FL), ΔD_(ABS-FR),respectively, which are calculated according to following expressions(6), (7) and (8), respectively:

    ΔD.sub.ABS-RR =D.sub.ABS-R ×(D.sub.TIRE -D.sub.G-RR)/D.sub.TIRE(6)

    ΔD.sub.ABS-FL =D.sub.ABS-F ×(D.sub.TIRE -D.sub.G-FL)/D.sub.TIRE(7)

    ΔD.sub.ABS-FR =D.sub.ABS-F ×(D.sub.TIRE -D.sub.G-FR)/D.sub.TIRE(8)

In these expressions (5) to (8) , D_(TIRE) is a diagonal wheel distanceand represented by D_(TIRE) =(L_(B) ² +L_(T) ²)^(1/2) ; each ofD_(G-RL), D_(G-RR) D_(G-FL), D_(G-FR) is a distance between each of theleft rear wheel W_(RL), the right rear wheel W_(RR), the left frontwheel W_(FL), and the right front wheel W_(FR) and the apparent centerof gravity position (G_(X), G_(Y)); and each of D_(ABS-F), D_(ABS-R) isa constant determined by a braking fluid pressure when the correspondingwheel has fallen into its slipping state, and is set such that D_(ABS-R)>D_(ABS-F). Thus, the amount of displacement toward the front wheels islarger than the amount of displacement toward the rear wheels.

Referring again to FIG. 8, if an angle formed by the straight line L andthe X-axis direction is represented by α_(RL), a following expression isestablished:

    α.sub.RL =tan.sup.-1 [{G.sub.Y -(L.sub.T /2)}/ {G.sub.X -(-L.sub.B /2)}]

and the amount ΔX_(ABS-RL) of displacement in the X-axis direction andthe amount of ΔY_(ABS-RL) of displacement in the Y-axis direction aredetermined by following expressions:

    ΔX.sub.ABS-RL =ΔD.sub.ABS-RL ×cosα.sub.RL(9)

    ΔY.sub.ABS-RL =ΔD.sub.ABS-RL ×sinα.sub.RL(10)

Likewise, when it has been detected that the right rear wheels W_(RR),the left front wheel W_(FL) and the right front wheel W_(FR) have beenfallen into their slipping states, the amounts ΔX_(ABS-RR), ΔX_(ABS-FL),ΔX_(ABS-FR) of displacement in the X-axis direction and the amountsΔY_(ABS-RR), ΔY_(ABS-FL), ΔY_(ABS-FR) of displacement in the Y-axisdirection are determined by following expressions:

When the right rear wheel W_(RR) has been fallen into its slippingstate:

    ΔX.sub.ABS-RR =ΔD.sub.ABS-RR ×cosα.sub.RR(11)

    ΔY.sub.ABS-RR =ΔD.sub.ABS-RR ×sinα.sub.RR(12)

When the left front wheel W_(FL) has been fallen into its slippingstate,

    ΔX.sub.ABS-FL =ΔD.sub.ABS-FL ×cosα.sub.FL(13)

    ΔY.sub.ABS-FL =ΔD.sub.ABS-FL ×sinα.sub.FL(14)

When the right front wheel W_(FR) has fallen into its slipping state,

    ΔX.sub.ABS-FR =ΔD.sub.ABS-FR ×cosα.sub.FR (15)

    ΔY.sub.ABS-FR =ΔD.sub.ABS-FR ×sinα.sub.FR (16)

Therefore, the amount ΔD_(ABS-X) of displacement in the X-axis directionand the amount ΔD_(ABS-Y) of displacement in the Y-axis direction aredetermined according to the above-described expressions (3), (4) and (9)to (16). This calculation is continued until all the four wheels arereleased from their locked states, or until the need for the ABS controlof both the rear wheels W_(RR), W_(RL) is eliminated, but the ABScontrol of both the front wheels W_(FR), W_(FL) is carried out.

The braking operation detecting section 35 delivers a high level signal,when it has been decided by the output from the depression forcedetecting sensor 3 that the braking operation is being conducted. Theswitch circuit 36 is switchable between a state in which the amountΔD_(ABS-X) of displacement in the X-axis direction and the amountΔD_(ABS-Y) of displacement in the Y-axis direction delivered from thereturn calculating section 33 when the output from the braking operationdetecting section 35 is of a low level, i.e., during non-braking, areapplied to the center of gravity position determining section 37, and astate in which the amount ΔD_(ABS-X) of displacement in the X-axisdirection and the amount ΔD_(ABS-Y) of displacement in the Y-axisdirection delivered from the return calculating section 33 when theoutput from the braking operation detecting section 35 is of a highlevel, i.e., during braking, are applied to the center of gravityposition determining section 37.

In the center of gravity position determining section 37, the center ofgravity position (G_(x) ^('), G_(Y) ^(')), after being forcedlydisplaced, is determined from the output G_(X), G_(Y) from the center ofgravity position calculating means 18 and the outputs ΔD_(ABS-X),ΔD_(ABS-Y) from the switch circuit 36 according to following expressions(17) and (18):

    G.sub.X.sup.' =G.sub.X +ΔD.sub.ABS-X                 (17)

    G.sub.Y.sup.' =G.sub.Y +ΔD.sub.ABS-Y                 (18)

The frequency counter 29 counts the frequency of slipping states intowhich the wheel is fallen. The frequency of slipping states is countedby increasing the number of slipping states when the same wheel has beensuccessively fallen into a slipping state and by decreasing the numberof slipping states when the different wheels have been irregularlyfallen into slipping states.

In the weighting coefficient determining section 30, a map of weightingcoefficient k determined in accordance with the frequency counted by thefrequency counter 29 is previously prepared, as shown in FIG. 9. And aweighting coefficient k is determined on the basis of this map. Thus,the weighting coefficient k is "1" when a frequency is "0" and becomeslarger than "1" as the frequency is increased. The weighting coefficientk is increased, as the slipping of the same wheel is successive.

In the time constant determining section 31, a map of time constant K,determined in accordance with the braking fluid pressure P_(B) at thetime of detection of the slipping state of the wheel by the slipdetecting means 17, is previously prepared, as shown in FIG. 10. And atime constant K is determined on the basis of this map. Thus, the timeconstant K is increased, as the braking fluid pressure P_(B), at thetime of occurrence of the slipping of the wheel, is increased. In otherwords, a condition in which the braking fluid pressure P_(B), at thetime of occurrence of the slipping of the wheel is large, is a conditionin which the friction coefficient of a travel road surface is relativelyhigh. Therefore, the time constant K is determined to be larger, as thefriction coefficient of a travel road surface is larger, i.e., on a roadsurface on which the wheel is more difficult to slip.

In the multiplying section 32, the weighting coefficient k is multipliedby the time constant K to provide a corrected time constant K'. Becausethe time constant K is determined such that it is larger, as thefriction coefficient of a travel road surface is larger, and theweighting coefficient k is determined to be larger, as the slipping ofthe same wheel is more successive, the corrected time constant K' islarger, as the friction coefficient of a travel road surface is larger,or as the slipping of the same wheel is more successive.

In the return calculating section 32, the outputs from the switchcircuit 36, i.e., ΔD_(ABS-X), ΔD_(ABS-Y) are received, and the amount ofΔD_(ABS-X) of displacement in the X-axis direction and the amount ofΔD_(ABS-Y) of displacement in the Y-axis direction are calculatedaccording to a linear delay function having the corrected time constantK' at a returning time to return the forcedly displaced center positionof gravity upon the completion of the braking operation. As the timeconstant K' is larger, the output from the return calculating section 32is smaller and hence, the returning speed becomes slower.

Thus, for example as shown in FIG. 11, the ΔD_(ABS-X), ΔD_(ABS-Y) areincreased from a time point t1 at which the slipping of a wheel occursduring a first braking operation, and the forced displacement of theapparent center position of gravity is carried out, and the returncalculation is carried out from a time point t2 at which the brakingoperation is completed. In this case, when the corrected time constantK' is "larger", ΔD_(ABS-X), ΔD_(ABS-Y) are decreased relatively slowly,as shown by a dashed line in FIG. 11, and when the corrected timeconstant K' is "smaller", the ΔD_(ABS-X), ΔD_(ABS-Y) are decreasedrelatively rapidly, as shown by a solid line in FIG. 11. Thedetermination of the returning speed by the time constant K' in thismanner ensures that if the slipping of the wheel during braking occursrelatively frequently, the ABS control is started in a condition inwhich ΔD_(ABS-X), ΔD_(ABS-Y) are not returned to "0", at a time point t3at which the slipping of the wheel occurs due to a second brakingoperation, so that ΔD_(ABS-X), ΔD_(ABS-Y) are increased again, and as aresult, a short time is only required until ΔD_(ABS-X), ΔD_(ABS-Y) arestabilized, leading to an enhanced responsiveness.

In the failure diagnosis sections 34_(X), 34_(Y), maps are establishedas shown in FIG. 12 by the corrected time constant K' and the outputsΔD_(ABS-X), ΔD_(ABS-Y) from the returning calculating section 33. Whenthe time constant K' is in a relatively large region (a region indicatedby oblique lines), it is decided that there is a failure, and an alarmis given, or another processing is performed. That is, a larger timeconstant K' indicates that the same wheel has been successively fallenseveral times into slipping states, or the wheel slipping has occurredon a road surface of a high friction coefficient where the wheelslipping is difficult to occur. Therefore, when the time constant K' islarger, it can be decided that a mechanism such as a tire or a brake isabnormal. In the failure diagnosis sections 34_(X), 34_(Y), a signalindicative of a failure is delivered. In this case, it is desirable thatreturn calculation of the ΔD_(ABS-X), ΔD_(ABS-Y) is stopped, ΔD_(ABS-X),ΔD_(ABS-Y) are maintained until the engine is stopped.

Referring again to FIG. 2, the vehicle speed V determined in the vehiclespeed calculating means 16 is supplied to a vehicle speed-correspondencecorrecting rate determining means 38, where a corrected rate C_(G1),corresponding to the vehicle speed V, is determined on the basis of amap previously established as shown in FIG. 13. The maximum value ofsuch correcting rate C_(G1) is "1".

The X coordinate G_(X) ^(') of the center of gravity position, afterbeing forcedly displaced, which has been determined in thegravity-center position forced-displacement amount calculating means 19,is supplied to a longitudinal acceleration-correspondence correctingrate determining means 39, where a correcting rate C_(G2) correspondingto an X coordinate G_(X) ^(') is determined on the basis of a mappreviously established as shown in FIG. 14. Here, the map is determinedin consideration of a weight balance of the vehicle, a tire size, andthe like, on the basis of the fact that the X coordinate G_(X) ^(')governs the longitudinal distribution of braking force and is dependentupon a longitudinal force/load characteristic of the tire, and themaximum value of the correcting rate C_(G2) is "1".

Further, the Y coordinate G_(Y) ^(') of the center position of gravity,after being forcedly displaced, which has been determined in thegravity-center position forced-displacement amount calculating means 19,is supplied to a lateral acceleration-correspondence correcting ratedetermining means 40, where a correcting rate C_(G3), corresponding tothe Y coordinate G_(Y) ^('), is determined on the basis of a mappreviously established as shown in FIG. 15. Here, the map is determinedin consideration of a weight balance of the vehicle and the like on thebasis of the fact that the Y coordinate G_(Y) ^(') governs thelongitudinal distribution of braking force and is dependent upon a sideforce/longitudinal force characteristic of the tire, and the maximumvalue of the correcting rate C_(G3) is "1".

The correcting rates C_(G1), C_(G2), C_(G3) determined in this mannerare supplied to an averaging calculation means 41, where a sum total ofthe correcting rates C_(G1), C_(G2), C_(G3) are divided by a correctingelement number, i.e., by 3 to provide an averaged correcting rateC_(GA1). The averaged correcting rate C_(GA1) is supplied to a gaincorrecting means 15, where a gain correction is carried out bymultiplying the first corrected total braking fluid pressure P_(T1) bythe correcting rate C_(GA1), thereby providing a second gain-correctedtotal brake fluid pressure P_(T2).

With the above-described the gain correction, as the correcting rateC_(GA1) is smaller, the braking force is weaker; the wheel is moredifficult to become locked; a cornering force is maintained, and thestability of the vehicle body is enhanced. Depending upon which of thebraking force or the stability is more important, the maps in FIGS. 13to 15 may be adjusted.

By adopting a corrected map corresponding to a brake depression force, aspeed of variation in brake depression force, or the like, it ispossible to enhance the brake feeling by a precise gain correction.Further, when a portion of each of the elements to be corrected is notcorrected, the correcting rate for such element to be corrected may beset at "1".

Referring to FIG. 16, the yaw control quantity calculating means 21includes a reference yaw rate calculating section 42 for calculating areference yaw rate Y_(B) as a target turn amount on the basis of thevehicle speed V determined in the vehicle speed calculating means 16 andthe steering angle Θ, detected by the steering angle detecting sensor10, a deviation calculating section 43, for calculating a deviation ΔYbetween an actual yaw rate Y_(B) detected by the yaw rate detectingsensor 11 and the reference yaw rate Y_(B), a control quantitycalculating section 44, for calculating a yaw control quantity Y_(E) bya PID calculation based on the deviation ΔY, a vehiclespeed-correspondence correcting rate determining section 45, fordetermining a correcting rate C_(G4) corresponding to the vehicle speedV determined in the vehicle speed calculating means 16, a longitudinalacceleration-correspondence correcting rate determining section 46, fordetermining a correcting rate C_(G5) corresponding to the longitudinalacceleration G_(SX) detected by the longitudinal acceleration detectingsensor 8, a lateral acceleration-correspondence correcting ratedetermining section 47, for determining a correcting rate C_(G6)corresponding to the lateral acceleration G_(SY) obtained by the lateralacceleration detecting sensor 9, an averaging calculation section 48,for averaging the correcting rates C_(G4), C_(G5) and C_(G6) to providean averaged correcting rate C_(GA2), a gain correcting section 49, forproviding a gain correction by multiplying the correcting rate C_(GA2)by the yaw control quantity Y_(E), and a combined calculation section50, for calculating a yaw control quantity combined with a control ofbraking fluid pressure on the basis of the total braking fluid pressureP_(T) determined in the total longitudinal force determining means 13and a yaw control quantity Y_(EC) corrected with the gain correction.

In the reference yaw rate calculating section 42, a yaw rate transferfunction is calculated for each of input steering angles θ, e.g., foreach of a plurality of vehicle speeds V set at intervals of 10 km/hr,thereby establishing a map as shown in FIG. 17. A reference yaw rateY_(B) is obtained by an interpolation in correspondence to the inputvehicle speed V. Thus, a suitable reference yaw rate Y_(B) is obtainedeven during a braking operation providing a large variation in speed.

In the vehicle speed-correspondence correcting rate determining section45, the correcting rate C_(G4), corresponding to the vehicle speed V, isdetermined on the basis of a map previously established, as shown inFIG. 18. In the longitudinal acceleration-correspondence correcting ratedetermining section 46, the correcting rate C_(G5), corresponding to thelongitudinal acceleration G_(SX), is determined on the basis of a mappreviously established, as shown in FIG. 19. In the lateralacceleration-correspondence correcting rate determining section 47, thecorrecting rate C_(G6), corresponding to the lateral accelerationG_(SY), is determined on the basis of a map previously established asshown in FIG. 20.

The correcting rates C_(G4), C_(G5), C_(G6), obtained in this matter,are supplied to the averaging calculation section 48, where the sumtotal of the correcting rates C_(G4), C_(G5), C_(G6) is divided by 3 toprovide an averaged correcting rate C_(GA2). In the gain correctingsection 49, the yaw control quantity Y_(E) is multiplied by the averagedcorrecting rate C_(GA2) to provide a gain-corrected yaw control quantityY_(EC).

In the combined calculation section 50, a calculation according to Y_(C)=Y_(EC) ×(2/P_(T)) is carried out on the basis of the gain-corrected yawcontrol quantity Y_(EC) and the total braking fluid pressure P_(T), anda yaw rate control quantity Y_(C), combined with the control of brakingfluid pressure, is delivered from the combined calculation section 50.

In the shared-load proportion calculating means 22, shared-loadproportions R_(FR), R_(FL), R_(RR), R_(RL) are determined by calculatingloads shared to the four wheels, after forced displacement of the centerof gravity position, calculating amounts of distribution of the yawcontrol quantity Y_(C) to the four wheels and further combining them.

More specifically, as a result of an apparent variation in center ofgravity position, a load WT_(F) on the side of both the front wheelsW_(FR) and W_(FL) is equal to (0.5×L_(B) +G_(X) ^('))×WT_(T) / L_(B),and a load W_(TR) on the side of both the front wheels W_(RR), W_(RL) isequal to (WT_(T) -WT_(F)). If the loads shared to the right front wheelW_(FR), the left front wheel W_(FL), the right rear wheel W_(RR), theleft rear wheel W_(RL), after variation in load, are represented byWT_(FR) ', WT_(FL) ', WT_(RR) ', WT_(RL) ', respectively, these sharedloads WT_(FR) ', WT_(FL) ', WT_(RR) ', WT_(RL) ' are represented byfollowing expressions:

    WT.sub.FL '=(0.5×L.sub.T +G.sub.Y ')×WT.sub.F / L.sub.T(19)

    WT.sub.FR '=WT.sub.F -WT.sub.FL '                          (20)

    WT.sub.RL '=(0.5×L.sub.T +G.sub.Y ')×WT.sub.R / L.sub.T(21)

    WT.sub.RR '=WT.sub.R -WT.sub.RL '                          (22)

If the amounts of distribution to the right front wheel W_(FR), the leftfront wheel W_(FL), the right rear wheel W_(RR) and the left rear wheelW_(RL), after variation in load, are represented by Y_(CFR), Y_(CFL),Y_(CRR), Y_(CRL), respectively, these distribution amounts Y_(CFR),Y_(CFL), Y_(CRR), Y_(CRL) are represented by following expressions:

    Y.sub.CFR =Y.sub.C ×{WT.sub.FR '/(WT.sub.FR '+WT.sub.RR ')}(23)

    Y.sub.CFL =Y.sub.C ×{WT.sub.FL '/(WT.sub.FL '+WT.sub.RL ')}(24)

    Y.sub.CRR =Y.sub.C ×{WT.sub.RR '/(WT.sub.FR '+WT.sub.RR ')}(25)

    Y.sub.CRL =Y.sub.C ×{WT.sub.RL '/(WT.sub.FL '+WT.sub.RL ')}(26)

Further, if the shared loads WT_(FR) ', WT_(FL) ', WT_(RR) ', WT_(RL) ',and the distribution amounts Y_(CFR), Y_(CFL), Y_(CRL), Y_(CRL), arecombined to find shared-load proportions R_(FR), R_(FL), R_(RR), R_(RL),they are as follows:

    R.sub.FR =(WT.sub.FR '+Y.sub.CFR)/WT.sub.T                 (27)

    R.sub.FL =(WT.sub.FL '-Y.sub.CFL)/WT.sub.T                 (28)

    R.sub.RR =(WT.sub.RR '+Y.sub.CRR)/WT.sub.T                 (29)

    R.sub.RL =(WT.sub.RL '-Y.sub.CRL)/WT.sub.T                 (30)

Thus, the sum total of the shared-load proportions R_(FR), R_(FL),R_(RR), R_(RL) is always "1".

The shared-load proportions R_(FR), R_(FL), R_(RR), R_(RL), determinedin the shared-load proportion calculating means 22, are supplied to thebraking fluid pressure calculating means 23_(FR), 23_(FL), 23_(RR),23_(RL), where target braking fluid pressures P_(FR), P_(FL), P_(RR),and P_(RL), as target longitudinal forces for the respective wheels, arecalculated for every wheel brakes by multiplying the second totalbraking fluid pressures P_(T2) by the shared-load proportions R_(FR),R_(FL), R_(RR), R_(RL), respectively, and the modulators 2_(FR), 2_(FL),2_(RR), 2_(RL) to which the drive means 24_(FR), 24_(FL), 24_(RR), and24_(RL) correspond are operated on the basis of the target braking fluidpressures P_(FR), P_(FL), P_(RR), P_(RL).

The operation of this embodiment will be described below. The targetbraking fluid pressures P_(FR), P_(FL), P_(RR), P_(RL) for the wheelbrakes B_(FR), B_(FL), B_(RR), B_(RL) are determined to control themodulators 2_(FR), 2_(FL), 2_(RR), 2_(RL) by determining the totalbraking fluid pressure P_(T), corresponding to the total braking forceexhibited by the wheel brakes B_(FR), B_(FL), B_(RR), B_(RL) mountedrespectively on the wheels W_(FR), W_(FL), W_(RR), W_(RL), bycalculating the shared-load proportions R_(FR), R_(FL), R_(RR), R_(RL)for every wheels W_(FR), W_(FL), W_(RR) and W_(RL), by distributing thesecond corrected total braking fluid pressure P_(T2), determined on thebasis of the total braking fluid pressure P_(T), in accordance with theshared-load proportions R_(FR), R_(FL), R_(RR), R_(RL). Therefore, evenif there is an unbalance in weight due to an increase or decrease inload or number of occupants, it is possible to maintain the stabilityduring braking and to reduce the nose dive, and the like.

In addition, the longitudinal and lateral accelerations G_(SX) andG_(SY) of the vehicle are detected to calculate the apparent amounts ofdisplacement of the center position of gravity of the vehicle. When thewheel is fallen into its slipping states during braking, the apparentcenter of gravity position of the vehicle is corrected, such that it isdisplaced toward a side away from the wheel in its slipping state on thestraight line connecting the wheel in its slipping state and theapparent center position of gravity of the vehicle as viewed in theplane, thereby determining the braking forces, such that the shared loadis larger on one of the front and rear wheel sides where more wheels arein non-slipping states when the slipping of the vehicle is detected.Therefore, while the total braking force, required by the occupant, isconstant, it is possible to exhibit the maximum braking force withinsuch a range.

Moreover, in the forced displacement of the apparent center of gravityposition, the constants D_(ABS-F) and D_(ABS-R), used in the expressions(5) to (8), are set to fulfill a relationship of D_(ABS) >D_(ABS-F),such that the amount of displacement toward the front wheels is largerthan that toward the rear wheels. Therefore, it is possible to enhancethe stability of the vehicle in such a manner that the frequency of theABS controls performed, upon occurrence of the slipping of the wheel orwheels during braking, is more on the front wheel sides than on the rearwheel sides.

Further, since the shared load proportions R_(FR), R_(FL), R_(RR),R_(RL) for every wheels W_(FR), W_(FL), W_(RR), W_(RL) are calculated onthe basis of the corrected shared load proportions WT_(FR) ', WT_(FL) ',WT_(RR) ', WT_(RL) ', it is possible when the distribution proportionsare changed, to disperse the increment at a proper distribution to thewheels having an increased distribution proportion.

A situation is supposed that, if the longitudinal acceleration G_(SX)and the lateral acceleration G_(SY) are increased, substantially all ofthe braking fluid pressure is applied to the wheel brakes on the wheelside of an increased load. In this case, if the characteristic of thetire is completely proportional to the variation in load and moreover,the braking force is obtained completely independently from thecornering force, there is no problem. But in practice, this is not true.That is, the increase in upper limit of tire generation force, due to anincrease in load, is gentle in an increased load region, and thecornering force and the braking force are in a strong correlation,wherein a larger braking force is obtained, when the cornering force islarger. In other words, if the vehicle is forcibly braked under such asituation, the cornering force is decreased abruptly. However, becausethe gain correction of the first corrected total braking fluid pressureP_(T1) is carried out on the bases of the X coordinate G_(X) ' and the Ycoordinate G_(Y) ', after forced displacement, the abrupt decrease incornering force is avoided.

Moreover, by adding the yaw control quantity Y_(C), determined on thebasis of the deviation between the reference yaw rate Y_(B) determinedon the basis of the steering angel Θ and the actual yaw rate Y_(A) tothe calculating elements of the shared load proportions R_(FR), R_(FL),R_(RR), R_(RL), the distribution of the target braking fluid pressuresP_(FR), P_(FL), P_(RR), P_(RL) is changed on the basis of the deviationbetween the target turn amount and the actual turn amount, and the sumtotal of the shared load proportions R_(FR), R_(FL), R_(RR), R_(RL) ismade constant. Therefore, a turning motion, appropriately correspondingto a stable longitudinal acceleration and a steering operation, can beprovided by distributing the braking fluid pressure while maintainingthe acceleration and deceleration of the vehicle constant.

Although the embodiment of the present invention has been described indetail, it will be understood that the present invention is not limitedto the above-described embodiment, and various modifications in designcan be made without departing from the present invention defined inclaims.

For example, although the brakes B_(FR), B_(FL), B_(RR), B_(RL) are ofthe same specification, and the total braking fluid pressure P_(T) isdetermined as corresponding to the total braking force in theabove-described embodiment, brakes which are not of the samespecification may be used. In this case, the total braking force may bedistributed at shared load proportions, and the braking force, afterbeing distributed, may be converted into a braking fluid pressure toperform a control of the brakes.

In addition, although the braking force, as the wheel longitudinal forcehas been described as being controlled for every wheels W_(FR), W_(FL),W_(RR), W_(RL) in the above-described embodiment, the present inventionis applicable to a vehicle in which the braking forces, at least on theside of the front wheels W_(FR), W_(FL) and on the side of the rearwheels W_(RR), W_(RL), are controlled collectively, and to a four-wheeldrive vehicle in which the driving force as the wheel longitudinal forceis controlled for every wheels W_(FR), W_(FL), W_(RR), W_(RL).

What is claimed:
 1. A process for controlling a wheel longitudinal forcein a vehicle in which longitudinal forces applied to a plurality ofwheels (W_(FR), W_(FL), W_(RR), W_(RL)) can be controlled at least on afront wheel side and on a rear wheel side, comprising the stepsof:detecting or determining a total longitudinal force which is a sumtotal of the longitudinal forces applied to the plurality of wheels(W_(FR), W_(FL), W_(RR), W_(RL)) in accordance with a total brakingfluid pressure; controlling the longitudinal forces applied to thewheels on the basis of a front wheel-side target wheel longitudinalforce and a rear wheel-side target wheel longitudinal force determinedby distribution of said total longitudinal force at a predetermineddistribution proportion; and detecting the presents of a slipping statefor each wheel, changing said distribution proportion by increasing anamount of said distribution proportion to one of the front and rearwheel sides in which more wheels are not in the slipping state, suchthat the target wheel longitudinal force is larger for the one side,wherein in changing said distribution proportion, the increased amountin said distribution proportion at a time when the front wheel-sidetarget wheel longitudinal force is increased is determined larger thanthe increased amount in said distribution proportion at a time when therear wheel-side target wheel longitudinal force is increased.
 2. Aprocess for controlling the wheel longitudinal force in a vehicle inwhich longitudinal forces applied to the plurality of wheels (W_(FR),W_(FL), W_(RR), W_(RL)) are controllable independently, comprising thesteps of:detecting or determining a total longitudinal force which is asum total of the longitudinal forces applied to the plurality of wheels(W_(FR), W_(FL), W_(RR), R_(RT)) in accordance with a total brakingfluid pressure; controlling the longitudinal forces applied to thewheels on the basis of a front wheel-side target wheel longitudinalforce and a rear wheel-side target wheel longitudinal force determinedby distribution of said total longitudinal force at a predetermineddistribution proportion; and changing said distribution proportion suchthat the target wheel longitudinal force is larger on one of the frontand rear wheel sides in which more wheels are in non-slipping states,when a slipping is detected in the wheels, wherein said changing saiddistribution proportion step further comprises the steps of:determiningloads shared to the wheels (W_(FR), W_(FL), W_(RR), W_(RL)) in a stoppedstate of the vehicle; providing apparent direction and amount ofdisplacement of a center of gravity position of the vehicle on the basisof a longitudinal acceleration and of a lateral acceleration of thevehicle; determining an apparent center of gravity position of thevehicle in accordance with said direction and amount of displacement;correcting the apparent center of gravity position of the vehicle whenslipping is detected in the wheels, such that said apparent center ofgravity position of the vehicle is displaced toward a side away from awheel in a slipping state on a straight line connecting the slippingwheel and the apparent center of gravity position of the vehicle, asviewed in a plane; correcting said determined shared loads on the basisof a corrected apparent center of gravity position of the vehicle; anddetermining the distribution proportion of the wheel longitudinal forcefor every wheel (W_(FR), W_(FL), W_(RR), W_(RL)) on the basis ofcorrected shared loads.